Musnad Abu Ya'la al-Mawsili 159:99

Abu Ma'mar narrated to us, Sufyan narrated to us, from Abu al-Zinad, from al-A'raj, from Abu Hurayrah; and from Hisham ibn Hujayr, from his father, from Abu Hurayrah—one of them [is the] narration—he said: Sulayman, peace be upon him, said: "I will certainly go around to one hundred women tonight, each of whom will give birth to a boy who will fight in the way of Allah." The angel said to him: "Say, 'If Allah wills.'" But he forgot. So he went around to them, and no woman among them gave birth except for one woman who brought forth half a boy. The Prophet, may Allah bless him and grant him peace, said: "By the One in whose hand is my soul, if he had said, 'If Allah wills,' every woman among them would have brought forth a boy who would fight in the way of Allah, and it would have been an attainment for his need."